The present invention relates to vehicle detection systems wherein a transmitter supplying a continuous wave signal is coupled to a receiver via sensing means so that the approach of a sensed vehicle produces an excursion increasing the amplitude of the envelope of the received signal.
Vehicle detection systems of the kind to which the invention relates are known.
In one known system, the sensing means is located beneath the road surface and comprises a transmitting coil forming part of the transmitter which is inductively coupled to a receiving coil forming part of the receiver. As a vehicle enters the zone of influence, the inductive coupling between the transmitting coil and the receiving coil is changed and causes a corresponding change in the level of the received signal.